Shedding light on the restart

Our reflection starts from basic considerations taken from the relevant literature, including in particular recent results about viral infection transmission and means to contain or eradicate it by UV–visible radiation as an effective antimicrobial agent [4], [5]. Before focusing on this point, we would like to recall two aspects concerning viral infections that, in our opinion, can drive the medical physicist towards specific conclusions on the best possible countermeasures with physical means. From the data available in the literature [6], [7] in a single cough we emit about 5000 particles whose dimensions vary from: (i) > 80 μm (6% particles) accounting for 99% of the total particle volume, readily falling to the ground; (ii) 40–80 μm (5%) aggregating into > 100 μm particles in case of high relative humidity (RH = 90%) while evaporating into < 40 μm particles otherwise; (iii) < 40 μm (89%) remaining long in air (up to few hours) due to their negligible weight compared to other driving forces [8], [9]. Likewise, in a single sneeze most particles are in the < 40 μm range (96%), while only 2% over 80 μm. This indicates the importance of addressing the problem of viral load transported by the smallest particles in air, especially in closed spaces such as hospitals, offices, shops and public transport. The second aspect is the possible influence of environmental conditions on infection diffusion. Such conditions can be represented by physical variables (e.g. temperature, relative air humidity, solar irradiance) whose influence can be largely studied by physical modelling, like in the case of droplet dimensions, evaporation and permanence in the air and the factors controlling them. In many respiratory infection cases a dependence of infectivity on seasonal variables has been observed [1], [10], [11], [12], some authors suggesting a correlation with seasonal variation in vitamin D serum concentration [13]. In our view, these data are consistent with the sure increase in ambient sunlight irradiance and in particular of its UV-violet components [14]. Starting from these observations, we would like to emphasize that UV-violet light in general may have a direct antimicrobial effect depending upon wavelength and dose, and that sunlight irradiance in the UV region is certainly a well-known seasonal phenomenon especially for temperate latitudes [14].